The present invention generally relates to xDSL communications. More specifically, the invention relates to active filters for analog front ends (AFEs) of XDSL communication systems.
With the advancement of technology, and the need for instantaneous information, the ability to transfer digital information from one location to another, such as from a central office to customer premises, has become increasingly important.
In a digital subscriber line (DSL) system, data is transmitted from a central office to customer premises via a transmission line, such as a two-wire pair, and is transmitted from the customer premise to the central office as well, either simultaneously or in different communication sessions. The same transmission line might be utilized for data transfer by both sites or the transmission to and from the central office might occur on two separate lines. In its most general configuration, a DSL card at a central office is comprised of a digital signal processor (DSP) which receives information from a data source and sends information to an AFE. The AFE serves as the interface between an analog line, such as the two-wire pair, and the DSP. The AFE functions to convert digital data, from the DSP, into a continuous-time analog signal when processing downstream data. Conversely, the AFE serves to convert an analog signal to digital data when processing upstream data.
As an important part of the aforementioned system responsible for proper transmission and reception of data in a broadband network, the AFE performs multiple functions in addition to converting a digital signal into a continuous-time analog signal, and vice versa. However, the functionality of the AFE is particular to the specific DSL application considered, wherein factors such as signal bandwidth, data rate, data reach, signal quality, power budget, and different applicable standards determine the optimum AFE.
In the receive path of the AFE, one fundamental component is a receive filter that may be configured to attenuate a strong echo signal so that the weaker receive signal may be later amplified without amplifying the noise produced by the echo. Unlike the transmit filter, the cut-off frequency of the receive filter is not extremely critical as the sole purpose of the receive filter is to attenuate the echo signal. What is critical however, is the attenuation in the stopband and the SNR in the passband. The specific configuration of the receive filter may vary with the particular application that will be utilized such as ADSL or VDSL.
Asymmetrical DSL (ADSL) is a popular service provided for residential customers, due to the asymmetrical nature of data usage, wherein more data flows towards the customer in the downstream direction, in comparison to the upstream data flow. The larger downstream data flow is generally attributed to accessing Internet Web sites. When a Web site is accessed, a relatively large amount of data related to the Web page is transmitted downstream to a browser located in a customer""s PC. The bulk of the upstream data is generally attributable to e-mail traffic directed towards other Internet users, and this constitutes a smaller data volume than the one traveling downstream.
Very High bit-rate DSL (VDSL) is similar to ADSL but provides large amounts of bandwidth with speeds up to about 52 Mbps. In order to provide such high speeds, a large bandwidth, as mentioned, is required. Typically, the VDSL frequency band ranges from 25 or 138 kHz to up to about 12 MHz. Unfortunately, the trade-off with high frequencies is the distance the signals can travel along a copper line. The development of Fiber to the Curb (FTTC) and/or Fiber to the Neighborhood (FTTN) provides for the use of VDSL by, essentially, reducing the length of the copper loop. Fiber cables, which can support high bandwidths over long distances, are provided from the CO to Optical Network Units (ONUs) located either at the curb of residences or the gateway to a neighborhood. The ONUs can convert the optical signals into electrical signals downstream and viceversa upstream. With the use of FTTN and or FTTC, the effective copper loop is reduced to below 4000 ft, which is the upper limit of VDSL. VDSL, for example, provides for the transmission of video signals over copper lines, which leads to applications such as digital television, Video-on-Demand, etc.
For VDSL applications at the CO, the receive filter must be high-pass to keep in line with the receive bandwidth. Based upon other considerations, such as ADC requirements, system requirements, etc., a 3rd order elliptic high-pass filter may suffice for VDSL applications at the CO. This type of filter has been used before but can be improved.
One aspect that can always use improvement in filter design is the power in which the filter requires (provided an active filter) to perform the necessary operations. Minimizing power consumption is often desirable because it can reduce the operating cost of the communication services provider utilizing the equipment. Another aspect of filters, particularly receive filters in DSL communication systems, that can be improved is the level of attenuated noise. The more noise that is filtered out, typically, the better the filter. Noise, such as echo, is often detrimental to the ADC and the DSP of an xDSL communication system.
Therefore, there exists a continual need for improved active filters, particularly for receive filters for communication systems, that consume less power and can simultaneously provide better performance.
The present invention relates to improved filters and methods of filtering a signal for, in particular, communication systems. The filters in accordance with embodiments with the present invention can operate at a lower power level and perform better than presently known filters.
In this regard, a first embodiment of the present invention may be construed as an active filter that includes a summing amplifier having an input and an output, wherein the output of the summing amplifier provides the output of the filter. The filter also includes a first RC integrator having an input and an output and configured to emulate a first capacitance, wherein the output of the first RC integrator is coupled to the input of the summing amplifier, a second RC integrator having an input and an output and configured to emulate a second capacitance, and a third RC integrator having an input and an output and configured to emulate a third capacitance, wherein the output of the third RC integrator is coupled to the input of the summing amplifier. A fourth RC integrator is also included in the filter and has an input and an output and is configured to emulate an inductance. The input of the fourth RC integrator is coupled to the outputs of the first and second RC integrators and the output of the fourth RC integrator is coupled to the inputs of the first and second RC integrator. Finally, the filter also includes a selectable input stage having a first state and a second state. In the first state the input stage is configured to provide a DC-coupled non-capacitive input and in the second state the input stage is configured to provide an AC-coupled capacitive input.
In another embodiment, an AFE includes a high-pass receive filter for a communication system. The filter includes an AC-coupled capacitive input and a plurality of RC integrators. At least one of the plurality of RC integrators includes a damping resistor in parallel with a feedback capacitor and a switch for enabling the damping resistor, such that when the damping resistor is enabled, the at least one RC integrator is damped so as to reduce DC instability.
Yet another embodiment may be construed as a DSL interface card that includes an active high-pass filter located in an AFE of the DSL interface card. The filter includes a summing amplifier having an input and an output. The output of the summing amplifier provides the output of the filter. The filter also includes a first RC integrator having an input and an output and configured to emulate a first capacitance, wherein the output of the first RC integrator is coupled to the input of the summing amplifier, a second RC integrator having an input and an output and configured to emulate a second capacitance, and a third RC integrator having an input and an output and configured to emulate a third capacitance, wherein the output of the third RC integrator is coupled to the input of the summing amplifier. The filter also includes a fourth RC integrator having an input and an output and configured to emulate an inductance, wherein the input of the fourth RC integrator is coupled to the outputs of the first and second RC integrators and the output of the fourth RC integrator is coupled to the inputs of the first and second RC integrators. Finally, the filter also includes a capacitive AC-coupled input stage.
Another embodiment of the invention may be construed as an active high-pass filter that includes a summing amplifier having an input and an output. The output of the summing amplifier provides the output of the filter. The filter also includes an inductive RC integrator configured to emulate an inductor and a plurality of capacitive RC integrators configured to emulate capacitors of various capacitances. The plurality of capacitive RC integrators are selectively coupled to the summing amplifier and the inductive RC integrator so as to produce a desired transfer function. The damping coefficients of the plurality of capacitive RC integrators are selectively controlled to reduce DC instability in the filter.
Another embodiment of the present invention may be construed as an active high-pass filter for a communication system. The filter includes a selectable input stage having a first state and a second state. In the first state, the input stage is configured to provide a DC-coupled non-capacitive input and in the second state, the input stage is configured to provide an AC-coupled capacitive input. The filter also includes a plurality of RC integrators.
A representative method for effectively filtering a signal in accordance with embodiments of the present invention includes: providing an active filter configured with a first RC integrator, wherein the first RC integrator comprises a damping resistor for reducing the DC gain of the first RC integrator; and controlling the damping resistor, such that the damping resistor is either enabled or disabled.